There are numerous varieties of filler materials used in industry. Properties of such fillers, such as hardness or softness, particle shape, color, refractive index, inertness, economics, or density make them industrially useful. One such class of fillers are of siliceous composition. Siliceous compositions are those compositions containing silicon dioxide, SiO.sub.2, or one of its derivatives. Microcrystalline silica, diatomaceous earth, perlites, volcanic ash and the like are different types of siliceous compounds used to make fillers. These and other industrial minerals are generally mined and require further processing to create fillers.
Fillers of high and low value are used widely in such industries as plastics, paints, adhesives, inks, insulation and the like. The value of the filler is dependent of the degree of processing, supply of the material, physical characteristics such as particle size and suitability to the given application.
With respect to the production of paints and plastics, one valued property of a filler is its color, or degree of whiteness measured by the G.E. Brightness Standard. G.E. Brightness is a well-known standard used to measure brightness wherein a measurement of 100 indicates perfectly white and 0 is perfectly black. G.E. Brightness is measured by a machine manufactured by Photovolt.TM. of Indianapolis, Ind.
Currently available siliceous materials meet many of the criteria for use as a filler in the applications noted above but are unfavorable in many instances because of insufficient brightness. For example, fillers known as anti-blocking agents are used in the manufacture of plastic sheeting, bags, and thin films. In order to prevent adhesion of plastic film to itself, fine particle size fillers are added to provide a "bumpy" surface on the plastic, thereby reducing their tendency to adhere to each other. In the vast majority of plastic film products being produced, white fillers with no off-color tinting and a refractive index matching that of the plastic are required. Siliceous minerals generally have the proper refractive index to maintain film clarity, but impurities are often present in these fillers. These impurities affect the color and result in discoloration of the plastic.
Another example in which sufficiently bright white siliceous fillers or other minerals are needed is in the formulation of white paints and colored paints using a white "tint base." Paints are made with resins, solvents or water, inert fillers and extenders, and pigments or coloring agents. The majority of these siliceous fillers have poor levels of brightness and possess off-color tints. Due to the presence of these undesirable properties, these siliceous fillers can only be used at low usage levels in paints, despite their excellent resistance to weathering and availability in the proper particle size. White fine siliceous filler material, however, can find new applicability at higher usage levels and find acceptance as a material that can be used to extend the efficiency of the expensive white pigment titanium dioxide, instead of merely acting as a filler, if the siliceous material could be made brighter and whiter.
In view of the above disadvantages, attempts have been made to upgrade inferior quality fillers by trying to improve their degree of brightness, as measured by the G.E. Brightness standard. Usually these methods are too expensive or the other properties of the fillers are radically altered.
For example, in some instances, soda ash (sodium carbonate) is used to brighten such products, but normally very high temperatures are necessary to help brighten the fillers, and the products tend to agglomerate due to the fluxing nature of the chemical. Subsequent comminution of the fluxed material does not usually provide a good yield nor a proper particle distribution of the filler material sought. Some filler material is achievable, using this method, as a by-product during the manufacture of another, coarser, product. Such a method, however, clearly has a low yield.
Other known methods employ wet processes using solvents designed to leach discoloring impurities from the filler. Acids such as ascorbic, citric, sulfuric, and even phosphoric acid have been used to leach discoloring impurities. These processes, however, can be expensive and unwieldy due to disposal problems.
For example, it is known that in paper whitening processes, aluminum silicate clay (i.e., kaolin), which is a siliceous material, is leached with phosphoric acid. This process involves suspending kaolin in a phosphoric acid solution. This mixture has a high concentration of water and low concentrations of both phosphoric acid and kaolin. This process of whitening paper products is performed in a slurry without the use of heat. After some period of time the phosphoric acid/kaolin mixture is filtered out leaving the colorizing agents in the liquid. This method is proven to be economically inefficient because the liquid and colorizing agents must subsequently be disposed and are therefore wasted.
Another method of whitening siliceous compounds is to mix silica with approximately 30% phosphoric acid by weight of the silica. This mixture is heated at temperatures equal to and less than 500.degree. F. After heating the mixture, a solid cementitious material is produced. This cementitious material, however, does not possess a desirable consistency because the mixture is not evenly and finely divided.